Did you ever run an adventure with D&D 5th edition rules?
If yes you've almost surely realized that the classic fight with the "Final Boss" could have surprising ends.
With a couple of lucky rolls a 5 characters' party can easily get rid of a single monster, even if it has an higher CR than that expected for that party level.
Game mechanics are designed to let chances for a weak group facing a more powerful single opponent.
A turn based system, with a set number of actions for every turn, makes the "Final Boss" to take only one action while the party takes a number at least equal to characters' number.
I say "at least" because PCs can increase that number by bonus actions and an accurate use of some features.
The different number of actions make "Final Bosses" usually weaker than those in previous editions.
Designers tried to fix it by adding legendary and lair actions which increase the number of actions a monster can take in its turn.
I think they are very useful but they have also a limit. A very few monsters have them, and they are those considered mighty by game traditions: Dragons, Beholders, Liches, Vampires...
What if the "Final Boss" is anything less mighty and more common, such as a banditi leader, an orc chieftain, a wizard, etc..?
You have one of the Good Dungeon Master's skills to call upon, that of a game designer.
You have to reskin the "Final Boss" into something that is a good challenge for a final fight.
Let's how to do.
Boosting the Monster
Rise some stats such as AC, HP, attack bonus or average damage.
I think it's normal if the monster would be an unique member of its type. It's tougher or stronger or it has better weapons and armors.
Increase number of actions
Make it by:
- Legendary and Lair actions: pick what you prefer from MM and give them to it. You can alse create them by yourselves.
- Bonus Action: very few monsters have them. Again read through MM or PHB to find some which may be appropriate for your monster. For example give Rogue's Cunning Action to a Goblin Boss to let it Strike and Run every turn.
- Multiattacck: increase attack numbers. Even if you're not increasing the number of actions you'd make your monster more dangerous.
Virtually any character can cast spell in
D&D 5th edition.
Why not to give that feature to you monster?
Give it some cantrips and spell slots.
Choose spell cast as a bonus action, always to increase the number of actions or choose spells which have an area of effect or multiple targets; with a single spell the monster will affect the whole party.
A DM's common mistake.
PCs kill the monster then find its loot, which contains magic items.
If anyone has a magic item and knows its powers it would use it.
Wands, armors and other items boost monster's defence and firepower.
If you're running a low magic campaign or if you don't want to give away too much magic to PCs then opt for single-use item.
Just give to the monster a potion of flying and a potion of supreme healing to make a challenge a lot harder.
Basta un mostro con una Potion of Flying e una di Supreme Healing per aumentare di molto la difficoltà della sfida.
It's a powerful creature. It should be normal for it to have bodyguards, henchmen, minions, apprentices, lieutnants or lesser monsters.
Have at least a couple of them, even if they have a very low CR.
You will give more protection to the "Final Boss", will increase the number of actions and will consume more party resources.
- Let my army be mountains, rocks, trees and birds in the sky...
Charles the Great
Terrain is important in a battle since Sun-Tzu's Art of War time.
Create traps for the party.
Make the terrain difficult for PCs but not for the monster.
Create natural situations lethal for the party but not for it. For example make the fight in a cave that has poisonous gas coming from holes in the walls, which forces PCs to mare a saving throw every turn while it has no effect on the monster.
Create places where the monster can hide or find protection.
Take away the Final Boss
You don't have a final boss if you don't want it.
You can swap with a group of lesser monsters or some other kind of challenge (riddles, puzzles, ability checks, etc..).
Use at your will one or more of those advices.
You'll change from time to time your last battle, giving a surprise to your players and making them satisfied.
It's your goal, as usual.